Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. A specification of the ion species, doses, and energies is referred to as an ion implantation recipe.
In conventional ion implantation, ions are extracted from a plasma source and are typically filtered (e.g., for mass, charge, energy), accelerated and/or decelerated, and collimated through several electro-static/dynamic lenses before being directed to a substrate. By contrast, in plasma-based ion implantation, a substrate is immersed in plasma. A negative voltage is applied to the substrate and ions are extracted through a subsequent sheath between the substrate and plasma.
Several types of plasma sources exist, such as capacitively-coupled plasmas (CCPs), inductively-coupled plasmas (ICPs), glow discharges (GD), and hollow cathode (HC), to name a few. Of these examples, ICPs are typically better suited for conditions of interest in ion implantation because of lower electron temperature and higher electron density when compared to CCPs. An example of an ICP is radio frequency (RF) plasma.
A cross-sectional view of a typical radio frequency plasma doping system (RF-PLAD) 100 is depicted in FIG. 1. The plasma doping system 100 includes a plasma chamber 102 and a chamber top 104. The chamber top 104 includes a conductive top section 116, a first section 106, and second section 108. The top section 116 has a gas entry 118 for a process gas to enter. Once the process gas enters the gas entry 118 of the top section 116, it flows on top of a baffle 126 before being evenly distributed in the chamber 102. The first section 106 of the chamber top 104 extends generally in a horizontal direction. The second section 108 of the chamber top 104 extends from the first section 106 in generally a vertical direction. A planar coil antenna 112 having a plurality of turns wraps around the second section 108. A helical coil antenna 114 having a plurality of turns typically sits on the first section 106 and surrounds the second section 108. The first and second sections 106, 108 are typically formed of a dielectric material 110 for transferring RF power to the plasma inside the chamber 102.
An RF source 130, e.g., an RF power supply, may be electrically connected to at least one of the planar coil antenna 112 and the helical coil antenna 114 by an impedance matching network 132 that maximizes power transferred from the RF source 130 to the RF antennas 112, 114. When the RF source 130 resonates RF currents in the RF antennas 112, 114, the RF antennas 112, 114 induce RF currents into the chamber 102 to excite and ionize process gas for generating a plasma in the chamber 102.
The geometry of the first and second sections 106, 108 of the chamber top 104 and the configuration of the RF antennas 112, 114 are chosen so that a uniform plasma is generated. In addition, electromagnetic coupling may be adjusted with a coil adjuster 134 to improve uniformity of generated plasma.
A platen 124 is positioned in the chamber 102 below the baffle 126. The baffle 126 may be grounded or floating. A target wafer 120 is positioned on a surface of the platen 124, which may be biased by a voltage power supply 128, so that ions in generated plasma are attracted to the target wafer 120.
Referring to FIG. 2, a conventional substrate or target wafer 120 is shown. Upon energetic ion bombardment on the wafer 120, secondary electrons are typically produced. For example, a 10 keV BF2+ ion produces approximately 6 secondary electrons upon impact with a silicon (Si) wafer. Thus, for every positive ion implanted, a +7 charge results on the wafer 120. At low pressures (e.g., a few mTorr), most of these electrons are transparent to bulk gas or plasma and may end up colliding with the baffle 126 to cause sputtering or heating up. At higher pressures (e.g., several tens of mTorr), an appreciable fraction of these secondary electrons may collide with bulk gas resulting in ionization and/or dissociation. Consequently, bulk density distribution and implant dose are greatly affected by unconfined secondary electrons.
In view of the foregoing, it would be desirable to provide a technique for confining on a wafer secondary electrons in plasma-based ion implantation to overcome the above-described inadequacies and shortcomings.